Historical Rules · 16 July 2010
Rule 901.13b
Official source text from the 16 July 2010 Rules release
If a player would leave the game and that player leaving the game would reduce the number of turn markers in the game, that player first ceases to be a planar controller (but no other player becomes a planar controller), then that player leaves the game. The face-up plane card that player controlled is put on the bottom of its owner's planar deck. No player is considered to have planeswalked.